1. Field of the Invention
The present invention relates to an electron gun assembly improved in its resolution and capable of keeping its focus grade uniform, and more particularly to an improvement of an electron gun assembly disclosed in U.S. Ser. No. 223,332, now U.S. Pat. No. 4,897,575.
2. Description of the Related Art
Electron gun assemblies which are called the three electron gun type are usually employed in the color cathode ray tube and most of them belong to the electron gun assembly of the in-line type in which three electron guns are aligned on a line. In the case of the color cathode ray tube apparatus provided with the electron gun assembly of the in-line type, inhomogeneous magnetic fields are used to deflect electron beams. More specifically, deflecting magnetic field of the pincushion type is used as horizontally deflecting one and deflecting magnetic field of the barrel type as vertically deflecting one to cause the three electron beams to be self-converged on a point above the screen and focused on the screen. The color cathode ray tube of this self-convergence type can reduce the amount of current used to contribute to enhancing its quality and capacity, and it has become the most popular color cathode ray tube today.
In the case of the color cathode ray tube apparatus of the self-convergence type, however, it has been found that deflecting magnetic fields of non-uniformity causes the resolution to be lowered at the pheripheral region of the screen in the color cathode ray tube. The section of the electron beams is distorted as the electron beam is deflected, and the beam spot can be kept as a true circle at the center portion of the screen but it is changed at the pheripheral region of the screen, overlapping a low bright halo, longer in the vertical direction, upon a high bright ellipse-like core, longer in the horizontal direction. This causes the resolution to be lowered at the pheripheral region of the screen. This distortion of the electron beam spot is resulted from the fact that a deflection yoke applies inhomogeneous magnetic fields to the three electron beams, causing the electron beams in deflecting magnetic fields to be gently focused in the horizontal direction but sharply in the vertical direction.
An electron gun assembly capable of preventing the resolution from being lowered because of this distortion of the electron beam spot is disclosed in U.S. patent Ser. No. 223,332, filed July 25, 1988 now U.S. Pat. No. 4,897,575. In the case of this electron gun assembly, a main lens section is formed between focusing and final accelerating electrodes, one or plural intermediate electrodes are located between the focusing and the final accelerating electrode, and a resistor located adjacent to the electron gun serves to divide final accelerating voltage applied into those of a certain value which are applied to the intermediate electrodes. Further, those sides of the focusing and final accelerating electrodes which face the intermediate electrodes are formed as plates each having openings to form a quadrupole lens, and voltage which rises synchronous with the electron beams deflected is applied to the focusing electrode. The electron gun assembly arranged in this manner allows the resolution to be improved at the center region of the screen and it can be therefore incorporated into the color cathode ray tube of the common type in which the focusing voltage is fixed.
In the case of the electron gun assembly in which the main lens is designed as the quadrupole lens, however, the vertical diameter of the electron beam is mainly adjusted while leaving its horizontal diameter almost not adjusted when adjustment is made relative to focusing voltage.
Upon adjusting focusing voltage, therefore, a compromise must be made between adjustments of beam diameter in the vertical and horizontal directions. This makes it impossible for the electron gun assembly to give full scope to its original capacity.
The conventional technique of dividing the focusing electrode into at least two pieces and using the quadrupole lens between the focusing electrodes to adjust the vertical focusing of electron beams independently of the horizontal focusing thereof is used in some industrial fields, but interference is strong in the vertical and horizontal directions in the case of this technique. In other words, when the focusing of electron beams is adjusted in one or vertical direction, the focusing of electron beams which has been adjusted in the other or horizontal direction must come to be adjusted again. Further, the focusing of the electron beams is adjusted by two potentials of high voltage. This causes conventional parts such as the stem pins and focus pack to be exchanged.
U.S. Pat. No. 4,672,269 proposes providing a variable resistor connected to the earthed terminal of a resistor outside the tube to achieve focus adjustment, but the focus adjustment is carried out using one potential in this case and this patent says nothing about adjusting the focusing of the electron beams in the vertical direction independently of adjusting it in the horizontal direction.
U.S. Pat. No. 3,995,194 discloses an electron gun assembly of the so-called try potential type. The focusing of electron beams is adjusted using two potentials in the case of this electron gun, but its main lens system is of the rotationally symmetrical type and it teaches no concept of adjusting the vertical diameter of the beam spot independently of the adjustment of horizontal diameter of the beam spot.
When focus adjustment is to be carried out, a compromise is only made between adjustments in the vertical and horizontal directions in the case of the conventional electron gun assemblies, as described above. In the case of the electron gun assembly in which the focusing of the electron beams is adjusted using two potentials, interference is strong in the both vertical and horizontal directions and this makes it difficult to adjust the focusing of the electron beams. Further, the focusing of the electron beams is adjusted by two potentials of high voltage, thereby causing conventional parts such as the stem pin and focus pack to be exchanged.